A study of bio-risk assessment, control and evaluation (BRACE) for indoor environment

Abstract

A better understanding of the exposure and deposition of bioaerosols in indoor environment is crucial for the improvement of indoor air quality (IAQ) and the minimization of the airborne infection. The information on exposure assessment of indoor bioaerosols in Hong Kong, however, is scare. Unlike the non-biological aerosols, the phenomenon of deposition of bioaerosols indoors is still unclear and not well investigated. Therefore, this study aims to provide important information on the exposure and control strategies of indoor bioaerosols through assessment, evaluation and investigation of the control factors of bioaerosols in the indoor environment of Hong Kong. Three principal objectives are (a) to assess the exposure of bioaerosols by investigating the abundance and composition of bioaerosols in various types of indoor environments in Hong Kong; (b) to evaluate the importance of bioaerosols in aspect of indoor air quality and identify the possible factors (e.g. ventilation) affecting the bioaerosols exposure in indoor spaces; and (c) to develop a method to investigate the deposition of some common bioaerosols in order to provide insights for the control of their occurrence in indoor environment. In this study, regional cross-sectional database of indoor airborne bacteria and fungi were developed by collecting samples from typical naturally ventilated residential apartments (N=103) and air-conditioned commercial offices (N=239). For the apartments, seasonal effects on the indoor airborne bacteria and fungi were revealed. Significantly higher average indoor airborne bacteria count (ABC) and significantly lower average airborne fungi count (AFC) were recorded in summer when compared with winter. Investigation of indoor airborne fungi at different locations within the same apartment revealed that the highest concentration was recorded in the kitchen, followed by the living room and the bedroom. For the offices, it was observed that both ABC and AFC in offices with excellent IAQ were significantly lower than those typical offices in Hong Kong. In general, Micrococcus, Staphylococcus, Aspergillus, Penicillium, Cladosporium are commonly found airborne bacteria and fungi in both apartments and offices. Higher indoor ABC in apartments and lower indoor ABC in air-conditioned offices, and lower indoor AFC in both apartments and air-conditioned offices were observed in Hong Kong when compared with other countries. This is the first study to use mathematical expression to reveal the importance of indoor bioaerosols assessment in the IAQ investigations of air-conditioned commercial buildings. It was found that AFC and ABC are the 2nd and 3rd ranked contributors for the unsatisfactory rates of IAQ in the offices. Furthermore, the positive effects of the existing IAQ policy in Hong Kong were observed in the significant decrease of indoor ABC and AFC in air-conditioned offices in the past decade. Furthermore, this study proposed several indices and models for the evaluation of the indoor bioaerosols. A relative index of fungal exposure (RIFE) was proposed to measure the relative exposure risk associated with an indoor environment compared with the outdoor one. A higher relative exposure risk was found indoors compared with that of the outdoors for both apartments and offices. Mathematical expressions on the relationship between the indoor bioaerosols levels and the temperature and relative humidity were developed; hence, under certain thermal environmental conditions, the excepted ABC and AFC can be rapidly estimated. In addition, a statistical model was developed for the prediction of pre-assessed unsatisfactory failure rate of air-conditioned public spaces over the local region. Several case studies on the evaluation of bioaerosols exposure were also conducted. The impact of indoor bioaerosols and some common air pollutants on the quality of life (QOL) of patients with Chronic obstructive pulmonary disease (COPD) was investigated. The patients' QOL were found to be strongly associated with the indoor environmental parameters recorded in their apartments, although the evidence of a causal relationship between them necessitated further research. Case studies investigation of the indoor bioaerosols exposure in the unoccupied office area and washroom under different ventilation conditions confirmed the strong effects of ventilation on bioaerosols exposure level. Therefore, an experimental facility was designed and built with the objective of understanding the deposition of bioaerosols in indoor environments. Five commonly isolated bioaerosols i.e. Micrococcus, Staphylococcus, Aspergillus, Penicillium, and Rhizopus were selected. The average diameter of bacterial cells and fungal spores ranged from 1 to 10 μm. It was found that the deposition of bioaerosols was significantly affected by the ventilation rate, mixing condition of the chamber, and the size of the bioaerosols. It was demonstrated that small-sized bioaerosols were chiefly influenced by the ventilation rate, while large-sized bioaerosols were mainly affected by gravity. A numerical model using the Eulerian-Lagrangian approach was developed to predict the deposition of bioaerosols in the chamber. The experiment results agreed with the numerical results quite well despite some discrepancies were observed. And the inclusion of the drag with particle sphericity was found to improve the prediction of bioaerosols deposition at low ventilation rate by using this model. The study provides some important information on the assessment, exposure evaluation, and control of indoor bioaerosols. Further work is needed to improve the numerical models for a better prediction and understanding the dynamics of bioaerosols in different indoor environments.